\[(b - 4)x = 9\]
\[x = \frac{9}{b - 4}\]
\[\frac{9}{b - 4} > 0,\ \ \ \ 9 > 0\]
\[b - 4 > 0\]
\[b > 4\]
\[Ответ:b \in (4\ ;\ + \infty).\]